I like straight one. To be honest whichever is less touched and natural on you, is the best for you. If you have naturally arched one, great. If you have naturally straight one, that is great too. More natural, more better.
I think it depends on the person. Some people can pull off both like she can. I think in the first picture she looks hotter but in the second one she looks cute. I noticed for myself I look better with a slight arch. I am not that pretty, so I do what I can to make myself look better. I've noticed that eyebrows definitely can make a difference for a person. I've noticed a difference with looking at pictures of myself. For some reason with straighter and thicker brows my face looks chubbier. With thinner and arched it looks thinner. It's weird how they can really make a difference.
I think it depends on the persons face because I've seen girls who are prettier with one verses the other just because one is more flattering towards their face shape. As for me I've done both and arched brows makes me look more "sexy" in a way but I look meaner on the other hand I look nicer and happier with straight brows